The present application relates to imaging systems, particularly diagnostic imaging systems. It finds particular application in conjunction with an integrated three-dimensional rotational angiographic (3DRA) and computerized tomographic (CT) diagnostic imaging system and will be described with particular reference thereto. However, it should be appreciated, that the present application may also find application in conjunction with other types of multi-modality diagnostic imaging systems. The present application-is particularly useful for visualizing blood vessel structures around and inside the skull.
For diagnostical purposes in a clinical environment, the separate visualization of the various represented tissue types when employing a radiological acquisition method can provide useful information about the clinical picture of a patient. With different radialogic methods being specialized on the visualization of one or a few kinds of tissues, the goal of displaying only useful information is normally achieved by choosing the adequate acquisition method. However, some tissue types tend to have similar properties regarding suitable acquisition methods, although their respective function inside the human body is completely different, e.g. blood vessels and bone structure. For the reason of different functions of the tissue concerned, it is desirable to represent them in a distinct manner, either by color coding according to tissue type or by masking all tissue types but the one the operator is interested in. The problem of overlapping property distributions for different types of tissue is particularly present in three-dimensional rotational angiography. In general, in 3DRA no absolute correct density values will be available due to insufficient calibration and reconstruction procedures. In this method, the segmentation of artery/vessel information in the 3DRA volume is hindered by the fact that the artery/vessel densities are in the same range as bone material, due to beam hardening caused by the skull. Furthermore, the high density data of the skull in the CT volume, which is typically used for orientation of the CT slices, prevents a clear view on the arterial structures, when both volumes are combined.
While Computer Tomography (CT) is known to provide high contrast between bony structures and soft tissue (e.g. skull as opposed to brain), the use of three-dimensional rotational angiography (3DRA) for extraction of 3-D vessel information is hindered by the fact, that the intensity distribution of high intensity bone overlaps with the intensity distribution of contrast filled vessels. This effect is especially due to the phenomenon of beam-hardening caused by the skull, in combination with insufficient calibration and reconstruction procedures.
U.S. Pat. No. 5,832,134 to Avinash and Alyassin discloses a method for removing dominating structures for enhancing visualization of desired structures that circumvents the time consuming human-operator interaction. The basic idea of the application disclosed therein is to distinguish between highly connected regions on the one hand and more weakly connected regions. Regions, that are strongly connected correspond to bony structures, whereas regions, that are less strongly connected correspond to vessels. The introduction of this property makes available a new feature for an improved automatic segmentation of unwanted bone structure from the interesting vessel information. However, the method only interprets the inherently unsuitable data material provided by the three-dimensional rotational angiographic acquisition method in a different manner and does not revert to more suitable information sources. Furthermore, the morphological algorithms described therein, e.g. connectivity analysis, and voxel dilation, depend greatly on an optimal parameterization, which in turn is dependant from geometrical conditions, e.g. acquisition resolution or size of the object.